In conventional solenoid valves, the magnetic force of the permanent magnet is transferred through pole pieces onto an armature. The magnetic coil of the solenoid is arranged radially relative to the axis of the magnet because the flux of the permanent magnet also extends radially relative to the operating axis of the valve plug and through the armature. This type of construction requires relatively large dimensions for several reasons. First, only a fraction of the available circular cross-section of the armature is efficiently utilized for the magnetic flux. Second, the solenoid coil is not optimally utilized due to the open magnetic circuit which causes stray fields. The force of the permanent magnet is effective always in the closing direction in all positions of the valve plug. Stated differently, conventional structures of this type are monostable. If conventional structures are to be modified to become bistable, additional effort and expense must be made to achieve a bistable characteristic in which the valve plug or valve stem remains stable in each of its two end positions.
U.S. Pat. No. 4,299,252 (Reinicke) discloses an electromagnetically operated actuator boosted by a permanent magnet. The actuator operates a valve poppet 80 which is normally closed, so that the known valve is monostable. Reinicke's valve operating mechanism has an upper pole and a lower pole with such an arrangement of the armature, electromagnet, and permanent magnet that the valve is always returned into the lower valve closing position. For this purpose, the armature 22 is held in the valve closing position by the permanent magnet 48 and by the force of the Belleville springs 24, 26 which are effective only in the closing direction. To open Reinicke's valve the solenoid coil 36 is energized, whereby the magnetic flux through the upper pole is increased by the combination of the flux of the permanent magnet and of the electromagnet. Thus, the valve closing force of the lower pole and of the springs is overcome to open the valve. When the coil 36 is switched off again, the forces of the permanent magnet and of the Belleville springs again close the lower pole. Such known arrangement does not suggest a structure that is selectively either monostable or bistable.
U.S. Pat. No. 4,392,634 (Kita) discloses an electromagnetic valve that employs two springs 64 and 74. One spring 64 biases the valve stem 8 downwardly at all times. The other spring 74 merely fixes the solenoid coil 7 in position to reduce so-called valve chattering. Suggestions for constructing a valve so that it can be selectively operated in a bistable mode or in a monostable mode are not made by Kita's disclosure.